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A new contact lens based system has been developed that can monitor glucose levels in tears. The system can also be used to detect the emergence of glaucoma, a leading cause of blindness. These graphene based lenses are stretchy, comfortable and completely transparent; marking an advance in medicine and another exciting application for graphene and nanomaterials.
Diabetes monitoring currently relies on the familiar but painful pin-prick test, where a small amount of blood is extracted and tested for glucose concentration, normally from the finger. Although this method is accurate enough, it is invasive, inconvenient and does not provide continuous monitoring.
Glaucoma is one of the leading causes of blindness worldwide and a major risk factor is the pressure within the eyeball. Typically, the disease progresses without any symptoms; by monitoring the intra-ocular pressure, it could be treated earlier, leading to better outcomes.
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Graphene-silver Nanowire: Transparent and Flexible
Graphene is a super conductor consisting of carbon atoms arranged in a hexagonal lattice structure with the thickness of one atom. Due to its unique structure, graphene shows massive improvements in conductance, strength, flexibility and transparency.
Graphene-silver nanowire was chosen due to its high transparency and durability as well as its favourable conductance. Its synthesis involves the transfer of graphene to random networks of silver nanowire. The lens has a high transparency (91%) and stretchiness (25%), allowing it to be worn just like regular contacts.
Glucose and Pressure Detection
Glucose is detected by using immobilised glucose oxidase, which produces gluconic acid and hydrogen peroxide (oxidised to oxygen, H+ and electrons). The production of electrons has a direct effect on resistance.
Pressure is measured by utilising a layer of silicone elastomer between layers of graphene-silver nanowire. Pressure changes result in a change in capacitance and inductance which can be monitored by the system.
By integrating the components into a RLC (resistance, inductance, capacitance) radio frequency circuit, outputs were recorded wirelessly. Glucose concentration was measured as a function of resistance in vivo in rabbit; and ocular pressure was measured as a function of inductance and capacitance in vitro in bulls’ eyes.
Ultraviolet–vis-NIR spectroscopy uses light in the visible and adjacent ranges to measure the reflectance of a material; a measure of its transparency. To analyse the optical integrity of the lens material, optical transmittance measurements were conducted by ultraviolet–vis-NIR spectroscopy, using a Cary 5000 UV-vis-NIR by Agilent. This system offers unsurpassed accuracy and delivers unmatched photometric range, linearity and lower noise.
Electron micrographs of the lenses were generated using SEM with a Hitachi S-4800. Scanning Electron Microscopy (SEM) can be used to generate surface images of nano-materials and here it was used to measure the sizes of the nanowires and to visualise the maximum curvature of the lens; producing accurate and clear images. The Hitachi S-4800 excels at ultra-high resolution scanning and has user-friendly GUI control as well as the ability to provide high resolution at low voltages.
The sensor was characterized with a semiconductor parameter analyser (Keithley 4200-SCS). The highest performance parameter analyser, it delivers synchronising current-voltage (I-V), capacitance-voltage (C-V) and ultra-fast pulsed I-V measurements, with real-time analysis.
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This research from the Ulsan National Institute of Science and Technology is groundbreaking in the fields of diabetes and glaucoma detection as well as ocular technologies. The system uses simple pyrene chemistry to anneal immobilised enzymes to the graphene layer: This means that it could be used in conjunction with a vast array of other purified enzymes to measure the concentration of disease biomarkers as well as the overall health of the body.
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